Gas-filled surge arrestor

ABSTRACT

A gas filled surge arestor has at least one external air back-up gap comprising two spaced electrodes(7a,2 or 7a,3), one (7a) of which has at least one sharp edge or corner (9) adjacent the other electrode (2 or 3). The said one electrode (7a) is resiliently urged towards the other electrode and is coated with an insulating material (8) which prevents the two electrodes from being in direct contact and hence short circuiting. The arrangement is such that, although the sharp edge or corner (9) on said one electrode has a covering of insulating material, it is also separated from the other electrode by an air gap.

The present invention relates to gas-filled surge arrestors or gasdischarge tubes comprising at least two electrodes defining a spark gapenclosed in a gas-filled housing. Such surge arrestors are designed tohave a normal breakdown voltage which is repeatable about apredetermined value. However if the interior of the surge arrestorbecomes vented to atmosphere, for example by sustained currentconduction and consequent physical damage, then the breakdown voltagewill become very much higher than the normal predetermined value andhence the surge arrestor is largely ineffective in performing itsdesired protective function. As an example a gas-filled surge arrestordesigned to have a normal breakdown voltage of about 150V may, when itsinterior is vented to atmosphere, have an erratic breakdown voltagebetween 2-4 kV.

Various proposals have been made to overcome this disadvantage. One suchproposal makes use of what is known in the art as "narrow-gaptechnology" in which the gap between the electrodes is made so smallthat the breakdown voltage of the surge arrestor is very similar whetheroperating normally or when its interior is vented to atmosphere. Howeverthis is a solution which is fraught with technical difficulties and isalso expensive to achieve. A further proposal is to provide an externalair back-up gap connected in parallel with the electrodes defining thegas discharge gap within the housing of the surge arrestor. Howeveragain it is found that the breakdown voltage of such a back-up air gapis higher than is desirable and a repeatable back-up gap breakdownvoltage is difficult to attain.

The present invention seeks to provide a gas-filled surge arrestorhaving an external air back-up gap which has both an acceptable value ofbreakdown voltage and a repeatable breakdown voltage.

According to the present invention a gas-filled surge arrestor isprovided with an external air back-up gap comprising two spacedelectrodes of which one has a sharp edge or corner adjacent the otherelectrode. The said one electrode may be coated with an insulatingmaterial at least over the region defining the sharp edge or corner.

According to a preferred form of the invention, said one electrode ispositively urged towards the other electrode and is coated with aninsulating material which prevents the two electrodes from being indirect contact and hence short-circuiting together. In this way thespacing between the two electrodes is defined to some extent by theinsulation between them. However, the electrodes are so shaped andarranged in relation to each other that although the sharp edge ofcorner on said one electrode has a covering of insulating material it isalso separated from the other electrode by an air gap. According to oneembodiment of the invention, said one electrode is planar and defines atleast one sharp corner and said other electrode defines a curvedsurface.

According to a further embodiment of the invention, said one electrodeis made of a spring material so that it is urged towards said otherelectrode and the arrangement is such that if the surge arrestor becomesoverheated the insulating material will soften or decompose, therebyallowing the two electrodes to come into contact and short circuit thegas discharge gap within the surge arrestor housing. In this way thestructure of the back-up gap also functions as a fail-safe device forthe surge arrestor.

The invention therefore also provides a gas-filled surge arrestor havingan external air back-up gap connected in parallel with the electrodesdefining the gas discharge gap within the housing of the arrestorwherein said back up gap is also constructed to function as a fail-safedevice which short circuits the gas discharge electrodes in the event ofoverheating of the surge arrestor.

In the arrangements according to the present invention one of theelectrodes of the back-up gap may also comprise one of the gas dischargeelectrodes of the surge arrestor.

The invention will now be further described, by way of example, withreference to the accompanying drawings, in which:

FIG. 1 is a perspective view of one embodiment of gas-filled surgearrestor according to the invention; and

FIGS. 2a and 2b are partial side and end views to a larger scale showingin detail the structure of the external air back-up gap.

Referring to the drawings a gas-filled surge arrestor or gas dischargetube which has two spark gaps comprises a central electrode 1 and twoend electrodes 2,3. The electrodes are held in spaced relationship todefine the desired gaps by means of annular ceramic members 4 and 5. Theelectrodes are secured to the ceramic members to define a housing filledwith an appropriate gas to assist in the correct functioning of thesurge arrestor as is well known in the art. The central electrode 1,which often forms an earth connection, is provided with a terminal pin6. A spring metal strip 7 extends along the body of the surge arrestorand is connected at its central region to the terminal pin 6. By virtueof the shaping and spring tension in the strip its ends 7a are urgedtowards the adjacent surface of the electrodes 2 and 3 but are preventedfrom electrical contact with these electrodes by virtue of a coating ofinsulating material 8. This can be seen clearly in FIGS. 2a and 2b. Thestrip 7 may be made of beryllium copper and the insulating material 8may be a polyurethane varnish. In practice the insulating coating 8 mayhave a thickness of some tens of microns, for example 20-40 microns, butas can be seen in FIG. 2b the thickness of the coating is much reducedat the sharp edges or corners 9 of the strip.

It has surprisingly been found that in the event of the interior of thesurge arrestor becoming vented to atmosphere, an air gap breakdown Doccurs between the sharp edge or corner 9 and the adjacent surface ofthe electrode 2 (or 3) at a very repeatable value and an acceptably lowvoltage level. For example, for surge arrestors designed to operate witha normal breakdown voltage within the range 150-250 volts, 210-310 voltsor 260-600 volts, the air gap breakdown repeatably occurred at a valueof 700-800 volts. It is believed that this low value of air gapbreakdown and its repeatability is probably due to the high degree ofionisation caused by the geometry of the gap and the sharp edge orcorner 9 formed by the strip 7. The coating of insulating material overthe sharp edge or corner is optional; in other words the region 9 couldbe exposed metal.

In this embodiment, overheating of the surge arrestor will cause thermaldecomposition of the polyurethane varnish thereby allowing the ends 7aof the strip 7 to move into electrical contact with the electrodes 2,3;so forming a fail-safe device which short circuits the internal sparkgaps of the surge arrestor. Moreover, this fail-safe mechanism will alsooperate to short circuit the surge arrestor in the case where continualdischarges D across the back-up air gap cause the insulating material 8to be removed due to the overheating caused by prolonged electricalarcing. A similar situation will again occur when the back-up air gap issubjected to a high a.c. voltage, e.g. of the order of 1000V rms ofvarying currents such that the heat generated by the arcing will besufficient to vaporize the insulating material 8.

Clearly other embodiments of the invention are possible. Thus, the twoelectrodes of the back-up air gap may both be rigid and both may beprovided with a sharp corner or edge. Other insulating materials may beused as are commonly employed in the art. The invention is obviouslyapplicable to surge arrestors having only a single spark gap as well asthose comprising more than two spark gaps.

We claim:
 1. A gas-filled surge arrestor comprising:a housing filledwith a gas; at least two spaced electrodes defining a gas discharge gapwithin said housing; and at least two spaced electrodes defining an airback-up gap external of said housing; wherein one electrode defining anair back-up gap has at least one sharp edge or corner adjacent the otherelectrode of said air back-up gap and a coating of an insulatingmaterial over at least the region defining said at least one sharp edgeor corner.
 2. A gas-filled surge arrestor according to claim 1 whereinsaid one air back-up gap electrode is resiliently urged towards theother air back-up gap electrode and the insulating material prevents thetwo air back-up electrodes from being in direct contact and hence shortcircuiting, said electrodes being so sharped and arranged in relation toeach other that, although the or each sharp edge or corner of said oneelectrode has a covering of insulating material, it is also separatedfrom said other electrode by an air gap.
 3. A gap-filled surge arrestoraccording to claim 2, wherein said one air back-up gap electrode isplanar and defines at least one sharp edge or corner and said other airback-up gap electrode has a juxtaposed curved surface.
 4. A gas-filledsurge arrestor according to claim 2, wherein said one air back-up gapelectrode is made of resilient material so that it is urged towards saidother air back-up gap electrode and the arrangement is such that, if thesurge arrestor becomes overheated, the insulating material softens ordecomposes, thereby allowing the two electrodes to come into contact andshort circuit the gas discharge gap.
 5. A gas-filled surge arrestorcomprising:a housing filled with gas; at least two spaced electrodesdefining a gas discharge gap within said housing; and at least twospaced electrodes defining an air back-up located externally of saidhousing and in parallel with said electrodes defining the gas dischargegap; said air back-up gap being also constructed to function as afail-safe device which short-circuits the gas discharge electrodes inthe event of overheating of the surge arrestor; wherein one electrodedefining said air back-up gap has at least one sharp edge or corneradjacent the other electrode defining the air back-up; and wherein saidone back-up gap electrode is resiliently urged towards said other airback-up gap electrode and said one electrode is coated with athermally-softenable or decomposable insulating material at least overthe region defining said at least one sharp edge or corner, saidinsulating material being arranged to prevent the two air back-up gapelectrodes from being in direct contact and hence short-circuitingunless the material softens or decomposes due to the surge arrestoroverheating thereby allowing said two air back-up gap electrodes to comeinto contact and short circuit the gas discharge gap, the air back-upgap electrodes being so shaped and arranged in relation to each otherthat, although the or each sharp edge or corner of said one electrodehas a coating of insulating material, it is also separated from saidother electrode by an air gap.